Printed circuit assemblies (PCA's) are typically tested after manufacture to verify the continuity between devices and pads on the assembly and to verify that components loaded on the PCA perform within specifications. Such printed circuit assembly testing is generally performed with automated in-circuit testers or ICT's or simpler manufacturing defect analyzers or MDAs, and require complex tester resources. The tester hardware must generally be capable of probing conductive pads, vias and vias on the assembly under test, in order to access the net to determine continuity. Sometimes a via on net is the only outer point accessible on the net.
In-circuit testers (ICT) have traditionally used “bed-of-nails” (BON) access to gain electrical connectivity to circuit wiring (vias, nets, pads) for control and observation capability needed for testing. This necessitates having access points within the layout of circuit nets that can be targets for ICT probes. Test access structures are usually circular targets with 28 to 35 mil (0.7 to 0.9 mm) diameter that are connected to vias on the printed circuit assembly. In some cases these targets are deliberately added test pads, and in other cases the targets are “via” pads surrounding vias already provided in the printed circuit.
Smaller diameter targets are increasingly difficult to hit reliably and repeatably, especially when a test fixture may contain several thousand such probes. It is always desirable to use larger diameter targets, but this is in fundamental conflict with the industry trend towards higher densities and smaller geometry devices.
Both test pads (i.e., flat pads with no holes) and vias (i.e., flat pads with a plated through hole for layer-to-layer interconnectivity, as shown in FIGS. 1 and 2) are used for probing most printed circuit assemblies at test. Many manufacturers use unplated bare copper as their assembly finish. This copper is covered with a thin organic protective film, such as OSP or OCC, to prevent oxidation prior to assembly. The protective film dissipates during the reflow process. Without covering the bare copper with solder prior to in-circuit test, conventional probing is difficult due to the buildup of copper oxides and the hard, thin nature of the outer layer of copper. As shown in FIG. 1, if the probe 18 misses the hole 15 and contacts the copper or metal test pad via 12 surrounding hole 15, the probe 18 often may penetrate the copper pad 12 and may damage the circuitry or short 16 to internal printed circuit assembly metallization layers 14.
One method to prevent this problem is shown in FIG. 2, in which the test pad via 12 is coated with solder paste 20 that is reflowed during surface mount assembly process. This process works well to promote electrical contact and prevent damage during test, but the solder paste 20 placed on bare copper via pads 12 often melts and runs into the plated through hole 15, leaving little or no solder on the surface 22 of the via pads 12 where the test probe 18 may make contact. The solder sometimes runs into the plated through hole 15 and plugs it, also resulting in the plug of solder being coated with a non-conductive coating of flux residue 24 and 26. This prevents good test probe contact (19) if the probe 18 goes into the hole 15. These problems are exacerbated when using lead-free solders, due to the different wetting properties and increasing popularity of OSP coated bare copper metallization in lead-free processing.